well this isn't exactly a knocker, but years ago we put cheap palm sanders to the underside of our bridge. strapped the corners with "plumbing strap" so they were tight to the bottom. 2 of them worked for a 12ft bridge, we also used a power drill with a piece of "flatbar" welded to a bolt ,chucked inside to slap a door to make it "slam" open & closed..you might could use that same type setup to work.
Slaughterwww.theslaughterhousehorror.com

i have used a air chisel on piece of metal wow is it load but you do need a actor. Also a chair massager motor both work great this dont have a sound effect but the vibration is really cool in the dark makes people think the are getting shocked

This is an idea I used for another prop that you might consider. Although it's not a knocker, it makes a rattle that is loud.

I built this out of 2x4s and mounted it under an empty wooden barrel. It made a lot of noise.

Its an octagon shaped ring. Inside there are metal bearings welded to spring-loaded thick metal wires. There are four of them in the picture but I think that's overkill, the thing made a ton of noise. Plus, if you remove three of the balls this will give you a nice vibration too. The only bad part is you will need to replace the boards. They wear down fast.

Each wire is attached to a motor shaft underneath the octagon plate. The plate is just a cut plywood sheet. This worked great for barrels, so maybe you can use it under your bridge.

One of my buddies at our haunt is a fire alarm tech. He came up with a couple fire alarm bell hammers (I believe are 24volt). You can find these online. I believe I posted this a while back. But the bell hammer simply strikes a steel plate (instead of the bell). He just mounted the electric hammer and the strike plate under the bridge. Extremely simple, virtually fail proof, and was LOUD and rattled the h-e-l-l out of the bridge he had it under!